Field Notes: The secret lives of basking sharks

Basking sharks will break you.

That’s an unfortunate lesson that’s been learned the hard way for centuries, among fishermen and researchers alike. It’s probably not for the reasons you imagine, because these are not your average sharks (although it is my duty as a shark biologist and conservationist to inform that you have more of a chance of being injured by a toilet than being “attacked” by any shark, including the infamous great white shark). Basking sharks (Cetorhinus maximus) are filter-feeders, which means that although they have miniscule teeth, they prefer to feed on large zooplankton such as copepods. They are one of three filter-feeding shark species worldwide, including the whale shark (Rhincodon typus) and the megamouth shark (Megachasma pelagios). Despite their diet of tiny organisms, all of these species can grow to incredible sizes. This is actually because they don’t have to hunt around for large prey items – plankton are abundant throughout the world’s oceans, and so filter-feeding organisms have constant access to a readily available food source. Basking sharks in particular can reach over 30 feet in length at maturity, although adults tend not to reach that size until 12-18 years of age. It’s hard to imagine seeing an animal that big in the water. I would know, because so far this field season, I haven’t seen one.

The basking shark opens its mouth wide to feed, filtering water and plankton through its gills. This behavior is often seen at the surface and is what inspired the name of this species. (Photo: Irish Basking Shark Study Group)

In theory, basking sharks can be found all over the world at temperate latitudes, preferring the oxygen-rich cool waters towards the north and south of the globe rather than the constant warmth of the tropics. Very little remains known about them. What we do know is that they are highly migratory, tracking warmer sea surface temperatures to follow plankton blooms. In doing so, these sharks can travel across the Atlantic Ocean (Gore et al., 2008) and the equator (Doherty et al., 2017; Skomal et al., 2009). However, they tend to aggregate (or gather) at certain times of the year in specific locations. These are known as “hotspots.” My research focuses on a basking shark hotspot along the coast of Ireland at Malin Head, which is the very northernmost tip of the entire country. This is where I have been, patiently waiting, since the beginning of July.

I first visited the Inishowen Peninsula (where Malin Head is located) last August, to get a feel for the field site and meet my collaborators at the Irish Basking Shark Study Group (IBSSG) and Queen’s University, Belfast. Basking sharks tend to aggregate here during July and August, the warmest months of the year. Warm, however, is relative – I often have to bundle in 3-5 layers of clothing for a full day on the boat, and on a nice day the water is barely about 60℉ (~15 ℃). Still, the sharks have been sighted here since the 19th century, and the IBSSG has been monitoring this population since the early 2000s. Often, the sharks can even be seen from land as they surface to feed, swimming along with their mouths wide open to filter the plankton through capture mechanisms in their gills (“gill rakers”). What’s interesting, however, is that we know almost nothing about what exactly they’re doing when they aren’t at the surface. This is important because a lot of current population estimates are based on surface sightings, but this method assumes that we would observe the shark if it were in the area. On sunny days, this is probably a fair assumption. The term “basking shark” comes from this habit of surface feeding, and there is evidence to suggest that the sharks are more likely to be at the surface when the sun is out (Emmett Johnston, pers. comm). However, at night or in other weather conditions, sharks are often out of sight to the human eye.

But why does this matter? A big reason is that this species is endangered in the northeast Atlantic region. Historically targeted by harpoon fisheries in Ireland, Scotland, and Norway for liver oil, basking shark populations were severely depleted in the 19th and 20th centuries. One genetics study suggested that the global population might be hovering around 10,000 individuals (Hoelzel et al., 2006), but specific numbers are still largely unknown. Most studies to date have examined the broad-scale movements of basking sharks (their migration and distribution patterns), and any attempts to look at this species through a fine-scale lens tend to focus on feeding behavior, because that’s what is easiest to observe. My goal is to delve a little bit further into other behaviors that can give a more complete picture about why basking sharks come to Malin Head. More importantly, I want to know if we think that they’ll behaviorally adapt to changing climate conditions by potentially leaving the area for good in the future.

This is my second summer working with Emmett Johnston (IBSSG) and Dr. Jonathan Houghton (Queen’s University, Belfast). Last year was relatively observational – I needed a feel for the animals and the field site before I could solidify my methods and my hypotheses. I then spent this past year writing grant applications and purchasing equipment. I use a method known as acoustic telemetry to examine movement patterns, attaching transmitters to the sharks that give off a sound signal underwater. I can then follow the sharks with a boat using a hydrophone onboard that receives the signal and points me in its direction, or I can deploy stationary hydrophones (receivers) underwater for longer periods of time that can record whenever a certain shark is in the area. Finally, we also attach accelerometers to the sharks, which give us measurements of the way the body moves with each behavior (kind of like a FitBit or the step counter on your phone).

Basking sharks are often sighted in small groups, although historically they’ve been observed in shoals of over 100 individuals. (Photo: Emmett Johnston, Irish Basking Shark Study Group)

 In all, my collaborators and I are working to get a better idea of where sharks go within the hotspot to exhibit certain behaviors – do they feed on small plankton patches, or is each hotspot area just one massive patch? Are all the sharks just here to feed, or do they socialize, even mate? Does the weather affect where they are in the water column? All of this information can generate a better understanding of why the sharks might come to Malin Head. In turn, our data may contribute to future predictions of whether they will continue to come here in the future. Temperate latitudes are parts of the world with clear changes in seasonality, and these are likely where we will see the first indicators of climate change. Indeed, studies already suggest that cold-water plankton species that used to reside in places like Malin Head have started to move north, replaced by warm-water plankton species (Hays et al., 2005). If the sharks are in Malin Head primarily to mate, or if they can feed on different plankton species, maybe this won’t matter. But what if the sharks are not able to behaviorally adapt? They’ll probably find somewhere else to go. There’s already evidence that basking shark distribution patterns are changing. Places where the sharks used to be abundant, based on records from the old fisheries, are no longer so. Researchers still aren’t sure whether this is because the population has not recovered or if the sharks have simply started inhabiting other areas. Such a lack of data makes any sort of policy or management for this species challenging. After all, it doesn’t make sense to establish a protected area around a basking shark hotspot if the sharks won’t stay in it for long.

The Fanad Head lighthouse overlooks a small bay at the mouth of Lough Swilly where basking sharks are occasionally sighted, just west of Malin Head. (Photo: Alexandra McInturf)

Given all of this speculation, I will admit that the absence of the sharks this field season is frustrating, if only because it generates more questions about this elusive species. Only 10 years ago, the sharks were allegedly gathering here in huge numbers, so many that you could “walk across them” according to some local sources. But basking shark presence has always been inconsistent throughout Ireland and the UK – fisheries reports and current studies indicate years of abundance intermingled with periods of total absence. Fortunately, I’ll be returning for the next 2-3 summers to complete my data collection here, and I’m curious to see what changes in that time. For now, I’m taking environmental measurements of the area, recording weather conditions, and filling my notebook with theories about why the sharks may not be here just yet. But of course, without the data we hope to collect, we can’t even really assume that they are absent. It’s possible that the sharks are simply beneath the waves, waiting for some signal or cue to emerge. If there’s one thing I’ve learned from my studies so far, it’s this: basking sharks will break you because once you catch shark fever, you’ll sacrifice almost anything (including your PhD) just to lay your eyes on one.


Alexandra McInturf is a second-year PhD candidate at UC Davis. If you’d like to keep up with her adventures studying basking sharks as a National Geographic Explorer, follow her blog on the website:

For information on her collaborators, Emmett Johnson (IBSSG) and Dr. Jonathan Houghton (QUB), click on the links below:

For information on basking shark history, including more details on the harpoon fishery, take a look at A Sea Monster’s Tale: In Search of the Basking Shark by Colin Speedie:


Doherty, P. D., Baxter, J. M., Gell, F. R., Godley, B. J., Graham, R. T., Hall, G., … & Speedie, C. (2017). Long-term satellite tracking reveals variable seasonal migration strategies of basking sharks in the north-east Atlantic. Scientific Reports7.

Gore, M. A., Rowat, D., Hall, J., Gell, F. R., & Ormond, R. F. (2008). Transatlantic migration and deep mid-ocean diving by basking shark. Biology letters4(4), 395-398.

Hays, G. C., Richardson, A. J., & Robinson, C. (2005). Climate change and marine plankton. Trends in ecology & evolution20(6), 337-344.

Hoelzel, A. R., Shivji, M. S., Magnussen, J., & Francis, M. P. (2006). Low worldwide genetic diversity in the basking shark (Cetorhinus maximus). Biology Letters2(4), 639-642.

Skomal, G. B., Zeeman, S. I., Chisholm, J. H., Summers, E. L., Walsh, H. J., McMahon, K. W., & Thorrold, S. R. (2009). Transequatorial migrations by basking sharks in the western Atlantic Ocean. Current biology, 19(12), 1019-1022.

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